1. Field of the Invention
The present invention generally relates to a magneto-optical disc recording and/or reproducing apparatus in which a signal is recorded on a magneto-optical disc by a so-called magnetic field modulation system. More particularly, this invention relates to a novel magneto-optical disc recording and/or reproducing apparatus in which, when a magneto-optical disc is chucked to a turntable, a spacer sheet is put on the magneto-optical disc at its side opposing an external, magnetic field generating coil, whereby upon playback the load on the revolution of the magneto-optical disc can be reduced, the consumed power can be reduced, the jitter caused by irregular revolution can be reduced and the service life of the external magnetic field generating coil and the spacer sheet can be prolonged.
2. Description of the Prior Art
A magneto-optical disc recording and/or reproducing apparatus has been proposed, in which a signal is recorded on a record medium by applying the heat energy of a laser beam and an external magnetic field to the record medium having a magnetic thin film which can be magnetized in a vertical magnetization mode and a recorded signal is read out from the record medium by detecting a reflected back light of a laser beam illuminated on the magnetic thin film owing the photomagnetic effect.
As a signal recording system for such a magneto-optical recording and/or reproducing apparatus, there have been developed a magnetic field modulation system in which a magnetic force of an external magnetic field is changed while the heat energy of a laser beam is kept constant and an optical modulation system in which heat energy of the laser beam is changed while the magnetic force of the external magnetic field is kept constant. It is to be noted that the magnetic modulation system is superior to the optical modulation system in that a so-called overwrite is possible wherein a recorded signal is erased and a new signal is recorded.
Incidentally, as the form of the record medium utilized by this kind of recording and/or reproducing apparatus, a disc-shaped record medium (hereinafter referred to as "magneto-optical disc") is known. When this magneto-optical disc is utilized, an optical pickup for illuminating the magneto-optical disc with a laser beam so as to form a beam spot and an external magnetic field generating coil for applying a magnetic field to the magneto-optical disc at its position corresponding to the beam spot are opposed to each other across the magneto-optical disc, and the optical pickup and the external magnetic field generating coil are unitarily moved in the radial direction of the magneto-optical disc.
Incidentally, since the magnetic force from a magnetic force generating source is in inverse proportion to the distance between it and the plane to which the magnetic force is applied, the distance between the external magnetic field generating coil and the magneto-optical disc must be kept constant so that the signal can be recorded stably by this kind of magneto-optical recording and/or reproducing apparatus.
However, if the magneto-optical disc is warped or if the axis of the rotation mechanism for the disc is inclined, there arises the problem such that the distance between the external magnetic field generating coil and the magneto-optical disc will be changed.
Accordingly, in order to keep the distance between the external magnetic field generating coil and the magneto-optical disc constant, it is proposed that the external magnetic field generating coil is brought in contact with the magneto-optical disc across a spacer sheet having an excellent slipping property. By way of example, a spacer sheet is secured at the central portion thereof to a chucking member which urges the central portion of the magneto-optical disc to a turntable, and when the magneto-optical disc is held between the turntable and the chucking member, the spacer sheet is brought in close contact with the magneto-optical disc at its surface opposing the external magnetic field generating coil and the external magnetic field generating coil comes in contact with the magneto-optical disc with a proper resilient force across the spacer sheet.
According to the above-mentioned proposal, even though the magneto-optical disc is warped and the axis of the turntable is inclined so that the magneto-optical disc is rotated while the surface thereof is vibrated, the distance between the external magnetic field generating coil and the magneto-optical disc can be constantly maintained the same as the thickness of the spacer sheet.
However, when the external magnetic field generating coil is brought in contact with the magneto-optical disc across the spacer sheet as described above, during the time that the magneto-optical disc is rotated, the external magnetic field generating coil exists as a load on the rotation of the magneto-optical disc constantly so that the load of the motor for rotating the magneto-optical disc is increased. As a consequence, the consumption of power is increased and the magneto-optical disc is not rotated regularly to thereby cause a jitter to occur in the reproduction. Further, friction occurs both in the external magnetic field generating coil and the spacer sheet, raising a problem of durability of this kind of magneto-optical disc recording and/or reproducing apparatus.